Delivering video content requires large amounts of bandwidth. Even when optical cables are provided with capacity for many tens of uncompressed channels, it is desirable to deliver even larger numbers of channels using data compression. Therefore, video compression methods (e.g., MPEG 2, H.264, Windows Media 9, SMTPE VC-9) are used to compress the video signals. With the advent of video on demand (VoD), the bandwidth needs are even greater.
In the MPEG-2 compression standard, a video stream is compressed into a stream of media units. Each media unit relates to a physical aspect of the compressed video stream, such as a group of pictures (GOP), frame, slice or macro-block. The media units may have different data sizes in bits according to the specific momentary content of the video stream (e.g., frames with many details or few details, similar adjacent frames or largely varying sequences of frames). The stream of media units is packaged into transport stream (TS) packets, generally having fixed sizes.
The strength of the compression may be selected as a compromise between reducing bandwidth and signal quality.
U.S. Pat. No. 6,054,943 to Lawrence, the disclosure of which is incorporated herein by reference, describes a video compression method in which two preprocessing modules are used to calculate lossless and lossy compression representations, respectively, for a video content. The compression varies seamlessly between lossy and lossless modes according to the attributes of the cannel.
Compression is generally a complex process that requires large processing resources. Therefore it is desired to perform the compression in advance, when possible. In addition, compression is often performed in a central location rather than in a plurality of remote locations. In these cases, the unit performing the compression does not have information on the current requirements of the channel and therefore the method of Lawrence does not achieve better compression than other compression methods.
There are networks in which each local transmission channel that carries one or more video streams is governed by a local statistical multiplexer that also adjusts the size of the video stream according to the available bandwidth (i.e., the bit rate which can be transmitted) of the channel. When necessary, the statistical multiplexer decompresses the video stream and recompresses the video stream at a lower rate. This, however, requires large processing resources and adds to the delay.
In order to reduce the processing load, there are video on demand (VoD) systems, e.g., multi bit rate (MBR) systems, in which two copies of a movie are stored. When a client requests the movie, the copy best suitable for the connection of the client to the VoD system is supplied to the client.
U.S. Pat. No. 5,768,535 to Chaddha et al., the disclosure of which is incorporated herein by reference, describes a scalable video compression method in which an image is repeatedly down scaled a plurality of times and error images describing the differences between the down-scaled images are generated. The images of each downscaling are encoded separately and transmitted on a transmission link. At a receiving end, a decoder extracts from the stream those portions that it can handle on its bandwidth.
U.S. Pat. No. 6,879,634 to Oz et al., the disclosure of which is incorporated herein by reference, describes a method of scaleable transmission of video.
U.S. Pat. No. 6,222,841 to Taniguchi, the disclosure of which is incorporated herein by reference, describes a system in which video streams include packets that can be discarded for stream shaping. When necessary, statistical multiplexers drop some or all of the discardable packets.
U.S. Pat. No. 6,108,382 to Gringeri et al., the disclosure of which is incorporated herein by reference, describes a method for transmission of video.
In some cases, video streams are accompanied by meta-data for use in performing fast forward and/or rewind operations during display at the client.
PCT patent publication WO 2004/025405, to Boyce Jill et al, filed Sep. 10, 2003, the disclosure of which is incorporated herein by reference, describes a video on demand server which includes separate stream multiplexers and a central statistical multiplexer.
U.S. Pat. No. 5,847,760 to Elmaliach et al., the disclosure of which is incorporated herein by reference, describes a variable bandwidth transmission system for broadcasting compressed video signals in which an encoder encodes each video frame (including a plurality of groups of pictures) at a maximal bit rate, half the maximal bit rate and at a quarter of the maximal bit rate. A transmitter controller selects which encoding is used according to available bandwidth. U.S. Pat. No. 6,674,796 to Haskell et al., the disclosure of which is incorporated herein by reference, describes a preliminary encoder which compresses video streams and during the compression generates encoding results, such as optimal motion vectors and inter/intra decisions. Encoders of a statistical multiplexer use the generated encoding results in the encoding of the video streams.